1. Field of the Invention
The invention relates to a stand-by drive operable by a crank handle in a drive unit actuable by means of an electric motor, in particular for sliding roofs of motor vehicles, wherein the drive unit comprises a gear which is connected to the electric motor, while an output part of the gear extending from the gear cooperates with the mechanism to be operated.
2. Background Discussion
Such stand-by drives, which in case of electric motor failure allow the actuation of the mechanism operable by the drive unit to be continued externally, have been known for a long time. Such a stand-by drive is mostly operated manually by means of a crank handle. In particular in sliding roofs of motor vehicles, the possibility of opening and closing the sliding roof has to be maintained. In view of the constructional space problems encountered particularly in the accommodation of electric motors for the actuation of sliding roofs, very small electric motors having a low torque are used at higher speeds in combination with an output gear having a great reduction ratio, mostly a worm gear, thus enabling the sliding roof to be operated at the desired speed with a high opening and closing force, respectively.
Since in conventional stand-by drives the output part--mostly a gearwheel, worm wheel or belt drive exiting from the gear and cooperating with the mechanism to be actuated--cannot be disengaged from the gear, and hence from the motor, for the purpose of a stand-by operation, difficulties in the manual stand-by operation arise from the high reduction ratio and the worm gear mostly used for achieving it.
Various kinds of stand-by drives are known. For example, it is known to provide the motor shaft of the electric motor with a worm of great lead and to provide the output part with a form-fit receiver for a crank handle. By very great efforts, the worm--and with it the entire motor shaft--can then be rotated through the gear and the worm wheel meshing with the worm. Such an arrangement has turned out disadvantageous in that very high manual forces are necessary, and due to its high lead, the worm no longer assures satisfactory self-locking. Thus, the sliding roof may slide unintentionally by itself, in particular owing to vibrations and jerks caused by rough ground when driving.
Another conventional stand-by drive provides a rotating lever to be positively inserted in an extension of the motor shaft and allowing the motor shaft to be rotated manually. A disadvantage of this arrangement is that the great reduction ratio of the gear requires numerous turns to be made in carrying out the stand-by operation. This accordingly results in long stand-by operation periods unacceptable in many applications, which is in particular also true for several applications different from sliding roofs. A further problem in this type of stand-by drive is the sealing of the motor, i.e. the sealing of the place where the extended motor shaft exits. Conventionally, the extended motor shaft exits on the motor side opposite to the gear connection side. The gear box is flanged to the motor and does not cause any sealing problems, as the motor shaft exiting there from the motor casing and penetrating into the gear box is completely enclosed by the gear box. Consequently, sealing problems only occur on the motor side where conventionally, the extension of the motor shaft designed for inserting the rotating lever is arranged.
A further conventional stand-by drive provides a friction clutch between the output part and the gear, the clutching force being exerted by disk springs. In case of stand-by operation, the disk springs can be released and, thus, the output part can be rotated independently of the gear and motor. However, in this arrangement it has proven disadvantageous that the rebending of the disk springs, i.e. the adjustment of the clutching force, entails considerable mounting work to be carried out in a garage. Further disadvantages result from the fact that after release of the clutch normally no position detection is any longer possible, and from the fact that the clutching force, and hence the torque transferable by the clutch, vary in the course of time.